Particulate solids are conventionally conveyed pneumatically using high velocity air or gas under negative pressure (vacuum). Such pneumatic conveying systems are used extensively in the oil, gas, chemical and petrochemical industries to discharge catalyst pellets from reactors into drums, either to allow the catalyst to be changed or to permit internal inspection or repairs to be carried out within the reactor. When discharging catalyst pellets in this way, it is important to avoid dust evolution, because the catalytic material often contains volatile and inflammable hydrocarbons, toxic elements, or residual precious metals. Some catalytic material may be pyrophoric, and extruded catalyst pellets (which are now widely used) are relatively fragile and can be easily damaged during pneumatic conveying, rendering them unfit for further use.
An example of a pneumatic conveying system for use in discharging particulate catalyst from a reactor is disclosed in U.S. Pat. No. 4,312,388. In the system described in the latter document, catalyst is discharged from a catalytic reaction vessel into a drum or series of drums via a chute and a normally limp transfer hose connected to a dip pipe which extends from the top of a hood for the drum down into the interior thereof, so that when the level of solids reaches the level of the discharge end of the dip pipe, the transfer hose becomes rigid and therefore detectable by the operator.
While this system is believed to work generally quite well, it is necessary to operate with the drum at an elevated pressure relative to the remainder of the pneumatic lines; otherwise the normally limp transfer hose would collapse.